Aerial Refueling Background:
Aerial refueling has significantly extended the range and mission effectiveness of modern military aircraft. Either of two refueling methods are used: the flying boom/receptacle method or the probe/drogue method.
The flying boom/receptacle method is usually used on one relatively heavy aircraft (called a receiver) at one time. Fuel flow takes place at a high rate so that a host can deliver to a receiver and it can take on loads of 100,000 lbs. or more in a matter of minutes. Due to the relatively large size of the host aircraft and the relatively heavy receiver, they have relatively slow rates of response. Thus, the boom operator of a tanker using the boom/receptacle method performs the precision "flying" while the pilot of the receiver aircraft maintains a formation position underneath and behind the tanker within a small "tolerance," usually about 10 feet.
The probe/drogue method, which is of particular concern in this patent, is a somewhat simpler technology than the flying boom/receptacle method because it requires a greater amount of piloting skill. In performing this method, a tanker aircraft unreels a hose approximately 50-100 feet behind the tanker. At the end of the hose is a funnel-shaped "basket" about three feet in diameter called a drogue. The center of the drogue has a refueling receptacle into which the probe of a receiver aircraft is inserted. When the probe is inserted into the drogue, fuel flows from the tanker into the receiver aircraft. Depending on the model of tanker aircraft, up to three drogues can be extended from the tanker and three receiver aircraft can be refueled at the same time, one from each of the opposite wingtips and one from a centerline refueling station. Flow rates of the probe/drogue method are less than the flying boom/receptacle method, but because it is used primarily with relatively lightweight, fighter sized aircraft, only small fuel offloads (i.e. 2,000 to 20,000 lbs.) at any single refueling are typically required, vice the 50,000 lb. plus offloads typically required by larger bomber or transport aircraft.
The probe/drogue refueling method is the more flexible method of the two refueling in flight methods since the probe/drogue method employs a pod that can be easily uploaded on a variety of aircraft, vice the flying boom method which requires a very large dedicated aircraft. In fact, it is even possible to install a refueling pod on a fighter, which then can refuel its wingman on a combat mission. In the probe/drogue method of refueling, C-130s, KC-135 and KC-10 class aircraft are typically used because of their endurance and fuel offload capabilities (i.e., 50,000 to 100,000 lbs.) depending on the mission radius and orbit time between the first and last offloads. The one current exception to this is the use by the U.S. Navy of the KA-6 as a host, which is a slightly modified A-6 attack aircraft. These tankers are used to "top off" formations once they reach altitude on the egress or provide an extra landing reserve prior to landing on an aircraft carrier.
Aerial refueling using the probe/drogue method is a demanding task for pilots. It requires visually observing the tanker in daylight or in darkness. In the case of darkness, special lighting on the tanker provides cues to visual position and depth. In operation, a receiver aircraft is maneuvered so that a probe, about six inches wide, is inserted into the mouth of a target drogue which is about three feet wide, to establish a connection with a receptacle structure and to maintain it until the refueling task is completed, which is normally between one and five minutes. Performance of this method is complicated by the fact that an aircraft's flying qualities change while it takes on fuel. The increase in weight requires the pilot of the receiver aircraft to increase pitch and power to generate more lift to maintain altitude and airspeed and maintain the probe/drogue connection. Another complication is that, when refueling a receiver aircraft, host aircraft actually orbit so that during all or part of the orbit, the host is in a banked turn, while the receiver must maintain its position relative to the tanker. It will be appreciated that, since there is a loss of lift in a turn, both the pitch and power of the receiver aircraft must be increased to maintain airspeed and altitude.
Background of Long Range/Endurance Unmanned Aerial Vehicles:
Quite apart from the technology of aerial refueling is the technology surrounding unmanned aerial vehicles (UAVs), sometimes referred to as remotely piloted vehicles. UAVs have been used as target drones for ground based surface to air missiles (SAMs), anti-aircraft artillery (AAA), and air to air missiles. As this technology has developed, UAVs were designed to simulate the capabilities of modern fighters or bombers as to speed, altitude, and endurance characteristics; for example, an unmanned aircraft may have a cruise speed of 400-500 mph, a cruise altitude of 20-40,000 ft., and at least an hour or more endurance.
Background of Sensor Technology:
Recently, a small number of these drones, unmanned aircraft or UAVs, have been fitted with sensors used to collect intelligence. Within the UAV spectrum is the typical battlefield reconnaissance UAV. These vehicles typically weigh 500 to 2,000 lbs. and used small gasoline powered engines with propellers and include a sensor package. The sensor packages often cost as much or more than the UAV. These packages can include various types of sensors: electro-optic (i.e., video camera), infrared (thermal imaging), radar (imaging), and signals intelligence (i.e., radio receivers). The sensors may be used one at a time or as a combination (depending on the payload and type of UAV). The intelligence collected is relayed in near-real-time (e.g. seconds or minutes from collection to exploitation) from the UAV to a ground site, via line of sight, or relayed transmission (which can use another UAV, aircraft, or satellite for the retransmission or data link).
As reconnaissance UAVs have continued to be developed to meet more demanding operational needs, an emphasis has been placed on increasing and improving payload (additional and higher quality sensors), endurance (more loiter time over target), range (which allows greater operational flexibility in chosing basing), stealth (harder to acquire and shoot down), and cruise speed (decreases time from take off to collection orbit).
The current state of the art in unmanned reconnaissance drones are the Tier 2 plus Teledyne Ryan "Global Hawk" and the Tier 3 minus Lockheed "Darkstar". The Global Hawk emphasizes payload, range, and endurance (2,000 lb. payload, 3,000 mile range, 24-hour loiter over target) while the Darkstar is very stealthy and has more limited operational characteristics (1,000 lb. payload, 500 mile range, 8-hour loiter over target). A major limitation of these (and all) UAVs is that despite the elimination of onboard aircrew members, a potential mission endurance of days or even weeks cannot be realized due to the limited fuel load carried at the time of takeoff.